The present invention relates to a process for cleaning gases, particularly coke oven gas, by removal of ammonia, hydrogen sulfide and hydrocyanic acid.
Various processes have been proposed for removing one or more of these substances from gases. In particular, oxidation processes are frequently used to remove hydrogen sulfide. This may be done by utilizing an alkaline fluid to absorb the hydrogen sulfide which is then oxidized by gasification of the solution with air, resulting in the formation of elemental sulfur which can be readily removed.
This process is successful in removing the hydrogen sulfide, and also hydrocyanic acid, from the gas but it has certain disadvantages. The alkaline fluids used are usually aqueous solutions of sodium or potassium salts or of ammonia, and other substances or additives are usually utilized to improve the transfer of the sulfur compounds or as activators. Thus, alkaline ammonia suspensions of metal hydroxides and various iron cyanide complexes are utilized. In some cases, arsenic compounds or anthraquinone disulfonic acids have also been used for increasing the transfer of the sulfur compounds. Furthermore, hydrocyanic acid which is usually present in coke oven gas is readily absorbed by the alkaline solution, which removes it from the gas by forming thiocyanates and possibly other complex compounds including related sulfur compounds. These undesirable products accumulate in the solution, in addition to the additives that have been intentionally added, and must be removed by diverting part of the flow of the solution at an appropriate point in the cycle for further treatment.
The solution thus diverted carries hydrocyanic acid and cyanide compounds, thiocyanates and other undesirable sulfur compounds such as thiosulfates and thionates. Processing this fluid has been the most difficult problem in the use of these known processes because the effluents and waste products which result must be disposed of in a non-polluting manner which satisfies the requirements that have been imposed for protection of the environment. Certain additives in the fluid can be recovered in some cases. Thus, if arsenic compounds are present, the fluid can be treated with sulfuric acid and the thiosulfates dissociate and precipitate arsenic sulfides to substantially completely remove the arsenic. Other undesirable compounds such as the cyanide compounds and sulfur compounds still remain in the fluid, however, and must be disposed of. Another environmental problem in the oxidation processes just described, particularly when the alkaline solution is an ammonia solution, is the absorption of ammonia by the air used for oxidizing the hydrogen sulfide. This air picks up large amounts of ammonia, and in some cases up to 50% of the ammonia content of the gas being cleaned is carried by the air discharged from the oxidizer. This may represent ammonia contents as high as 60 g/Nm.sup.3 and because of this high ammonia content the air has to be further treated before it can be discharged to the atmosphere.
Ammonia in the coke oven gas must be completely removed in the process of cleaning the gas and the disadvantages of the oxidation process just described may be avoided by combining it with the ammonia removal processes disclosed and claimed in my prior patent applications Ser. No. 367,532, filed June 6, 1973, and Ser. No. 485,232, filed July 2, 1974, both assigned to the Assignee of the present invention. These applications disclose a process for removing ammonia from gases by washing the gas with an inorganic acid solution, such as sulfuric acid or hydrochloric acid, which forms an ammonia salt solution and thus removes the ammonia. The salt solution is then mixed with a fuel gas to form a combustible mixture which is burned at a high enough temperature to decompose the ammonia to form nitrogen and water vapor which can be discharged to the atmosphere. The combustion process also results in recovering the acid anhydride which is recycled for reuse in the washing cycle. In the second above-mentioned patent application, acidic ammonium bisulfite is used as the inorganic acid which adsorbs the ammonia from the gas to be cleaned by forming an ammonium sulfite salt solution. This salt solution is regenerated to the ammonium bisulfite by treatment with sulfur dioxide, and a portion of the regenerated acid thus obtained is withdrawn and burned in the manner described above to provide the required sulfur dioxide.